Method of and apparatus for rendering objects visible in infrared rays



* March1 6,1937. Q N 2,074,226

METHOD F min APPARATUS Fofi' RENDERING' OBJECTS VISIBLE IN INFRARED RAYS File d may 21, I936 F i g I roundings.

Patented Mar. 16, 1937 METHOD OF AND APPARATUS FOR- REN- DERING OBJECTS VISIBLE IN INFRARED RAYS Josef Kunz and Franz Urbach, Vienna, Austria; said Urbach assignor to said Kunz Application May 21, 1936, Serial No. 81,090 In Germany May 18, 1935 12 Claims.

The invention relates to a method of and apparatus for rendering objects visible in infrared rays, and particularly for enabling objects which are covered by fog or are not visibly illuminated to be observed with the eye. For this purpose the invention makes use of the phenomena peculiar to theaction of infra-red rays on excited phosphors.

Phosphors have the property of storing up light energy and giving it ofl again spontaneously. in the form of visible light. .This light emission is influenced by the action of infra-red rays, but the nature of this influencing varies according to the nature of the phosphors used.

In the case of one group of phosphors the light emission is reduced by infra-red light, so that the area on which the infra-red rays fall glows more weakly than the spontaneously emitting sur- When the infra-red light used is of great intensity the darkening effected is preceded by a short period of increased brightness in the case of certain phosphors pertaining to this group. If the amount of light spent up to the attainment of complete darkening be measured it is found that this amount is very considerably less than the amount of light stored up in the course of excitation. The stored up light en ergy is thus to a considerable extent not given off again as light but destroyed in some other way. Phosphors pertaining to this group, in which the effect of infra-red rays is to reduce the illumination, are hereinafter termed phosphors of the extinguishing type.

In the case of a second group of phosphors infra-red irradiation has the efiect of intensifying the illumination. In the case of these phosphors the stored up light energy is mainly given off again in the form of visible light under the 'action of infra-red rays, and only to a very slight extent destroyed in other ways. At the same time the intensification of the light emission is the stronger the greater the intensity of the infra-red irradiation. As soon as the stored up amount of light has been completely driven out by the action of the infra-red rays the area irradiated appears dark. Phosphors in which the stored up light energy is driven out, in proportion to the intensity of the infra-red light used,

with the emission of light, are hereinafter termed phosphors of the exluminating type. I

In order to render visible objects in the dark or objects hidden in fog an infra-red real image of the objects is thrown, in accordance with the invention, upon an excited phosphorescent screen which is protected from the access of rays other than those proceeding from the objects under observation, but which can be viewed by the eye of the observer. Since the infra-red rays proceeding from or refracted by the object observed are generally speaking of varying intensity from 5 I one point to the other, they produce upon the phosphorescent screen an image of the object which is modulated by the infra-red rays.

When a phosphor of the extinguishing type is employed, there results a negative image of the object. If on the other hand a phosphor of the exluminating type be employed, there is obtained a positive image. The positive images produced with the employment of phosphors of the exluminating type show greater contrasts so that the detailsof the object can be more readily recognized than in the case of negative images. This is more particularly the case when the objects move while under observation.

A large number of phosphors are known which are suitable for use in the method according to the invention. For the production of negative images there may be employed a phosphor prepared from zinc sulphide with an addition of copper, and for the production of positive images a phosphor prepared from zinc sulphide with the addition of small quantities. of manganese.

Other phosphors suitable for the presentpurpose are those prepared from the sulphides. oxides, and selenides of the alkaline earth metals, and furthermore of zinc, cadmium, and magnesium, with the addition of very small quantities of heavy metals such as antimony, bismuth, manganese, lead, and nickel. The rare earths may also be used as additions. If desired, fluxes such as calcium fluoride, or lithium phosphate may also be added. Many of these phosphors show excellent exlumination, for example in particular CaSPb, CaSSb, CaSNi, SrSBi, in the terminology introduced by Lenard, as also mixtures of such phosphors.

Further examples of suitable phosphors are the alkali halides, which may be used either with or without the addition of metals. The metals more particularly suitable for addition are silver, thallium, lead, copper, and manganese. An example of such a phosphor would be sodium chloride with an addition of silver, or potassium chloride with an addition of thallium. If the alkali halides are employed without the addition of metal the presence of disturbances in the crystal lattice isparticularly advantageous; Such disturbances can be brought about by heating to glowing temperature, allowing to set from a fused state, pressing or crushing, and other forms of mechanical or thermal treatment.

For the purpose of exciting the phosphors various kinds of rays may in general be used. In

the case of many phosphors, for example the calcium sulphide phosphors, visible light or longwave ultra-violet rays as emitted by a quartz lamp, will sufiice. For other phosphors, for example the alkali halide phosphors, it is better to use rays of shorter wave length, such as for rays emitted by radio-active substances also exert an exciting action upon almost all phosphors. In the case of some phosphors, for example the alkali halide phosphors, it is of advantage to excite them successively with difierent kinds of rays, for example first with cathode rays and then with visible light or ultra-violet rays, or first with light of very short wave length and then with light of longer wave length.

After excitation the phosphors emit light spontaneously. This emission is mostly dependent on temperature; it is almost always weaker and of longer duration at low temperature, and proceeds intensively but for short duration at high temperature. When the temperature is sufliciently low this spontaneous emission can be practically inhibited. However, the quantity of light spontaneously emitted, the total luminosity", is generally independent of temperature.

The luminosity of phosphors cooled to sufficiently low temperature can remainstored up for a considerable length of time. This condition is called the "lower momentary condition" of the phosphor. This cornditon varies for the different emission bands of the various phosphors. For example in the case of the a-bands of calcium oxide phosphors (to use Lenards terminology) this condition is obtained near room temperature. In the case of the a-bands ofthe calcium sulphide phosphors it is generally considerably below that temperature (approximately at the temperature of liquid air). In the case of selenide phosphors it is still lower. Phosphors which after excitation are vigorously phosphorescent even at room temperature, are called "phosphors of normal temperature characteristic". Phosphors which have to be heated above room temperature in order to render the emission visible are called phosphors of high temperature characteristic.

The influence of temperature on the emission. of the phosphor may be made use of to increase the contrast in the images obtained.

When the image is to be produced by exlumination of the phosphor, the spontaneous emission is inhibited either by using phosphors of high temperature characteristic at room temperature or phosphors of normal temperature characteristic cooled down to a sufficiently low temperature. In many cases it is advantageous to effect the excitation at higher temperatures than the exlumination.

It also has to do with the temperature behaviour of the phosphors that it is sometimes advantageous to interpose a rest period of a certain duration between excitation andexlumination, during which period part of the resulting excitation decays.

For the observing of negative images obtained with the employment of phosphors of the type in which the phosphorescence is extinguished by infra-red rays it is advantageous to heat the phosphor.

The exlumination of the phosphorescence is in the case of many phosphors a phenomenon which disappears after exposure to ultra-red rays, while in the case of other phosphors the exlumination persists after cessation of the exposure. For the purpose of observing moving objects there are used phosphors with strong exluminating capacity and short persistence", for example the Lenard phosphor CaSPb or suitably excited potassium or rubidium chloride. In many cases of observing stationary objects it can be desirable to use a phosphor of long persistence. A known phosphor of this type is the ZnSMn phosphor.

It is of advantage for the method according to the present invention to employ phosphors in which the total luminosity which can be obtained by exposure to infra-red rays (light quenchabie luminosity") is greater than that obtained b heating ("heat quenchable luminosity). Such phosphors are for instance rubidium chloride or potassium chloride.

According to the temperature characteristic of the phosphor it may be desirable to combine in one and the same apparatus the means serving for excitation and the means serving to produce the infra-red image, so that excitation and exluminating or extinguishing can take place in more or less rapid succession and, if desired, even simultaneously. In the case of phosphors of good storing capacity (phosphors of high temperature characteristic) it is generally better to separate the exciting and the exluminating processes both in space and in time.

It is also possible to look through the image on the phosphors, provided the phosphor layer be sufiiciently transparent for the light emitted; otherwise the image must be looked at. Observation can be facilitated by the provision of optical means, such as magnifying glasses, mirrors, or prisms.

Forms of construction of apparatus suitable for the observation of objects in infra-red rays, in

accordance with the present invention, are shown by way of example in the accompanying drawing, in which:

Fig. 1 shows a form of apparatus with a luminous screen permeable to the phosphorescent light, in longitudinal section.

Fig. 2 is a section of apparatus for observing the images in plan view.

Fig. 3 shows combined apparatus for exciting and observing, in horizontal section taken on the 1 line III-III of Fig. 4.

Fig. 4 is a vertical section taken on the line IV--IV of Fig. 3.

Fig. 5 shows a further form of observing apparatus combined with an exciting device, in section on the line V--V of Fig. 6.

Fig. 6 is a section taken on the line VI-VI of Fig. 5, and

Fig. 7 shows a detail in plan view.

The apparatus shown in Fig. 1 is similar in construction to a photographic camera the ground-glass screen of which is replaced by a phosphorescent screen I which is transparent to phosphorescent light. An objective lens 2 mounted in the adjustable front wall throws a real image of the object to be observed on the screen which may be observed through the eye-glass 3 mounted in the adjustable rear wall of the apparatus. In order to keep out light other than that proceeding from the object under observation acrea e there is provided between the screen and the obv jective lens a bellows t and between the screen and the eye-glass a bellows 5. The eye-glass is further equipped with a mounting 6 for the lighttight application of the eye.

In use, the luminous screen is excited by means of a suitable source of light, for example a quartz lamp, placed in front of the objective lens, and e of the exluminating type is employed, the image obtained will be negative or positive.

The device shown in Fig. 2 is designed for observing a screen in plan view. An image of the object is projected on a phosphorescent screen 9 with the aid of a lens l and a prism B. For the purpose of observing the image there is provided the eye-glass it and the prism l. The phosphorescent layer is applied to a disc which is formed in onepiece with a cylindrical sub-structure l3 accommodated in a vessel it. When positive images are produced the vessel It is filled with a coolingmedium, for example liquid air, in order to keep the phosphor at so low a temperature that the spontaneous emission of light is inhibited. For the production of negative images it may be expedient under certain circumstances to heat the phosphor by means of a heating medium placed in the vessel it, with a view to increasing the luminosity of the parts not encountered by infra-red rays.

The apparatus shown in Figs. 3 and 4 is equipped with means for excitingthe phosphor. It differs from that shown in Fig. 2 by the fact that the optical arrangement consisting of the prism M and lens i5 for the production of an image of the object upon the phosphorescent screen it is disposed at an angle to the opticalarrangement (prism ill and eye-glass i5) serving for the observing. There are further provided a cylindrical diaphragm i8 rotatable about its axis and provided withthree apertures it, 2 and ti, and a suitable light source 22 for the excitation of the phosphor, the light from which source passes through a collecting lens 23 and is deflected by a prism 2t on to the phosphorescent screen it.

The relative position'of the slots is, 20, ii is such that the cylindrical diaphragm prevents the access of the light from source 22 to the phosphorescent screen It when the slots l9 and 20 are in the position in which it is possible to observe. the projected image. When on the other hand the slot 2! registers with the collecting lens 23 of the light source both the lens i5 and also the eye-glass l5 are covered by the diaphragm it. When the slotted cylindrical diaphragm is is progressively rotated about its axis periods of excitation of the luminous screen will thus alternate with periods during which it is possible to observe the image.

In the apparatus shown in Figs. 5 to '7 a circular disc 25 carrying four phosphorescent screens 26, 26', 26" and 26 is rotatable by means of a knob 29, about a vertical axis 21, within a closely encasing housing 28. To one side of the axis 21 there is mounted on the housing a lighttight box 30 containing the means required for producing an image of the object and for ob- 7 serving this image, and to the other side of the axis there is provided a box ti containing a light source 32 suitable for the excitation of the phosphor. The optical arrangement serving for the production oi an image of the object consists of a mirror 33 and a system of lenses 34. The image formed on the phosphorescent screen is observed in the mirror 36 through an eye-glass it.

In the bottom of the casing 28 there is inserted 'within the box 30 a solid cylindrical metallic body 37 which is disposed coaxially with the lenses M of the objective. The metallic body it? dips in the cooling liquid, for example liquid air, placed in the vessel 33 and is thereby retained at a very low temperature.

When the phosphor is cooled to a very low temperature it is expedient to take steps to ensure that the deposition of hoar-frost on the phosphorescent screen and the optical means of the apparatus be prevented. For this purpose moisture attracting substances, such as phosphorus pentoxide, may for example be placed in the box 3%, accommodated in small containers 39. The air enclosed in the box 30 is thereby kept dry. It is further of advantage to cover the window aperture it provided in the top of the casing 38 above the metallic body 31! with a hollow glass body M likewise containing small vessels 39 filled with moisture-attracting substances. A hollow glass body of this nature proves preferable to a glass plate, because its upper wall is at a conscreens is effectually prevented by the fact that' the disc 25 carrying the screens is enclosed in a very closely fitting casing 28 having but very small air capacity.

If the phosphor is to be cooled during excitation there is provided beneath the exciting device a cooling arrangement of similar construction to that acting upon the observed screen.

In general it is suitable to maintain the phosphor at a higher temperature during the exciting than during the observing.

through In the stationary periods one of I the phosphorescent screens, for instance the screen 26, is excited by the light source 32, while the diametrically opposite phosphorescent screen 26" is observed. Both screens 26 and 26 are at the same time cooled. The screen 26 is prefer ably not cooled to so low a temperature as the screen 26". which was excited during the preceding stationary period, loses a certain amount of its luminosity in the meantime and is at the same time brought to a slight y higher temperature, because it is outside both sets of cooling means. In the next stationary period the image of the object to be observed is thrown upon this screen 26'. During this latter process it rests upon the cooling body 31 and is cooled down to so low a temperature,

that the spontaneous light emission is practically inhibited.

The phosphorescent screen 25',

If desired, provision can be made for the phosphor to arrive at the position in which it is observed after having already been cooled down to a very low' temperature.

The apparatus shown in Figs. 5 and 6 has the advantage of enabling the objects to be observed for any desired length of time almost uninterruptedly, in spite of the fact that the total luminosity capable of being stored by the excitation of the phosphor is limited and that the spent luminosity has to be renewed by excitation of the phosphor.

We claim:

1. Method of rendering visible objects situated in the dark or covered by fog, which comprises producing an infra-red real image of the objects upon an excited and superficially extended phosphor, the stored up light energy of which is expelled mainly with light emission by infra-red light in proportion to the intensity of the latter, and maintaining the phosphor during the observation of the said image at so low a temperature that the spontaneous emission is practically inhibited.

2. Method of rendering visible objects situated in the dark or covered by fog, which comprises producing an infra-red real image of the objects upon an excited and superficially extended phosphor the stored up light energy of which is expelled mainly with light emission by infra-red light in proportion to the insensity of the latter, and cooling the phosphor during the observation of the said image to so low a temperature that the spontaneous emission is practically inhibited.

3. Method of rendering visible objects situated in the dark or covered by fog, which comprises producing an infra-red real image of the objects upon an excited and superficially extended phosphor of high temperature characteristic, the stored up light energy of which is expelled mainly with light emission by infra-red light in proportion to the intensity of the latter, and maintaining the phosphor during the observation of the said image at so low a temperature that the 7 spontaneous emission is practically inhibited.

4. Method of rendering visible objects situated in the dark or covered by fog, which comprises producing an infra-red real image of the objects upon an excited and superficially extended phosphor of low temperature characteristic, the stored up light energy of which is expelled mainly with light emission by infra-red light in proportion to the intensity of the latter, and cooling the phosphor during the observation of the said image to so low a temperature that the spontaneous emission is practically inhibited.

5. Method of rendering visible objects situated in the dark or covered by fog, which comprises producing an infra-red real image of the objects upon an excited and superficially extended phosphor, the exlumination of which shows short persistence and the stored up light energy of which is expelled mainly with light emission by infrared light in proportion to the intensity of the latter, and maintaining the phosphor during the greater than the heat quenchable luminosity and the stored up light energy 01 which is expelled mainly with light emission by infra-red light in proportion to the intensity of the latter, and maintaining the phosphor during the observation of the said image at so low a temperature that the spontaneous emission is practically inhibited.

7. Method of rendering visible objects situated in the dark or covered by fog, which comprises exciting a superficially extended phosphor, the light energy 01 which is expelled mainly with light emission by infra-red light in proportion to the intensity of the latter, the said phosphor being maintained during the exciting step at so low a temperature that a spontaneous light emission is practically inhibited, producing an infra-red real image of the objects on the phosphor, and maintaining the phosphor during the observation at so low a temperature that the spontaneous light emission is practically inhibited.

8. Method of rendering visible objects situated in the dark or covered by fog, which comprises exciting with two different kinds of rays a superficially extended phosphor the light energy of which is expelled mainly with light emission by infra-red light in proportion to the intensity of the latter, producing an infra-red real image of the objects on the phosphor, and maintaining the phosphor during the observation at so low a temperature that the spontaneous light emission is practically inhibited.

9. Apparatus for rendering visible objects situated in the dark or covered by fog, comprising in combination a light tight chamber, a phosphorescent screen housed in the said chamber, means for cooling said screen, means for producing an image of the objects on the said screen, and means for observing said screen.

10. Apparatus for rendering visible objects situated in the dark or covered by fog, comprising in combination a casing housing a rotatable disc, phosphorescent screens provided on said disc, an enclosure mounted on said casing, means for producing an image of the objects on one'at a time of said screens, means for observing said screen with the image produced thereon, said image producing and observing means being provided within said enclosure, a further enclosure mounted on said casing, means within said further enclosure for exciting the said phosphorescent screens one at a time simultaneously with the observing of the screen with the image produced thereon, and cooling means for said screens.

11. Apparatus as claimed in claim 10, wherein between the screens acted upon by the image producing and exciting means respectively, there are arranged further screens on said rotatable 

